Hesitation or stutter when engine is cold...
 

'10 with Stillen Gen 2 intakes and catback, 22k miles.

The car recently developed a "stutter" or hesitation/jerking at just over 3k RPM-4K RPM (roughly) when the engine is cold. It will occur in any gear. Car has had all scheduled service, no warning lights on dash. At first I thought it could be dirty air filters (forgot about cleaning them at the 15k service), but I properly cleaned/reoiled them and it didn't help the stutter.

It does go away after the engine has warmed up.

I should also mention that it occurs when you're holding steady speed. Give it more gas to accelerate and the stutter goes away. It's most noticeable to me in moderate freeway traffic, when I just want to put the car in 5th gear and cruise along at 60-70mph. On surface streets, where I'm constantly changing speed and gears, it's rarely noticeable.

I've had the Stillen parts since roughly 5-6k miles with zero issues until 21k miles or so. No other mods, no service, no changes made to the car recently. It just sort of "happened". Same fuel, same driver, same driving style, absolutely no changes I can think of that might contribute to this issue.

Anyone had a similar issue? How do I resolve it?

6-speed manual, by the way.